1. Field
A method and system for detection of variations in plethysmographic oximetry.
2. Background
Pulsus paradoxus is a term referring to a systolic arterial pressure and pulse pressure that weakens abnormally during inspiration. It was first recognized in 1873 when an irregularity of the palpable pulse was observed while the heart sounds indicated that the cardiac rhythm was regular. It was found that the “irregularity” of the pulse resulted from a reduction in left ventricular stroke volume leading to an impalpable pulse during inspiration. Pulsus paradoxus may be symptomatic of various abnormalities including pericardial tamponade, worsening asthma, chronic obstructive pulmonary disease, congestive heart failure, pulmonary edema, chronic dyspnea, obstructive sleep apnea and tension pneumothorax. If left undetected, these disorders may result in deterioration or death of critically ill patients. Thus, early detection is essential.
In a healthy individual, arterial and venous blood pressure vary throughout the respiratory cycle. It is not uncommon to see an increase in blood pressure during expiration followed by a decrease in blood pressure during inspiration. Such fluctuations may occur due to the intrathoracic pressure changes generated during breathing. Although the exact physiology behind pulsus paradoxus may vary depending upon the disease it is symptomatic of, the exaggerated pressure decrease may generally be explained by the interrelationship between the changing intrathoracic pressure during the respiratory cycle and the two ventricular chambers of the heart. During inspiration intrathoracic pressure decreases which augments right heart filling, pulmonary vascular compliance, and right ventricular stroke volume while reducing left heart filling and output.
Pulsus paradoxus may be detected by monitoring changes in blood pressure throughout the respiratory cycle. Under normal conditions, an individual may experience a decrease in arterial blood pressure of less than 10 millimeters mercury (mmHg) during inspiration. An abnormality is identified where this pressure decrease exceeds 10 mmHg. Currently there are a variety of techniques available for detecting this pressure decrease during inspiration.
One such technique for detection of pulsus paradoxus requires gradually deflating a sphygmomanometer (blood pressure cuff) while listening for the onset of Korotkoff sounds (sounds resulting from arterial pressure waves passing through the occluding cuff) during normal respiration. The Korotkoff sounds will first be audible during expiration only, and after further deflation of the cuff, during inspiration as well. If the cuff is deflated more than 10 mmHg between detection of intermittent and constant Korotkoff sounds, pulsus paradoxus is said to be present. This method of detection is problematic for a number of reasons, not the least of which is that automated blood pressure monitoring recording is now standard throughout the health care industry and this technique is incapable of detecting respiratory fluctuations in arterial pressure since only one set of systolic and diastolic pressures are recorded. Even when manual sphygmomanometers are available, the operator has to be alerted to the possibility that pulsus paradoxus could be present and know how to perform the test. In addition, blood pressure values are subjective in that they are reliant upon the operator's ability to listen to the sounds while watching the fluctuations of the gauge. The only record is therefore the operators' hand-written description of a highly subjective test.
A second technique used in detecting pulsus paradoxus is by direct monitoring of arterial pressure with an indwelling intra-arterial catheter. This technique is more accurate than sphygmomanometry in detecting pulsus paradoxus because it results in a permanent recording of the arterial waveform and pressure and thus allows for an objective measurement. Due to its invasive nature, however, it is often painful to the patient and requires a highly trained health care provider.
Infrared photosensors used for pulse oximetry and plethysmography may be utilized for a third technique that may be used for detecting pulsus paradoxus. In this technique, changes in the intensity of an infrared (IR) beam passing through a patient's finger tip, toe, or earlobe are obtained to measure fluctuations in regional blood volume, a correlate of blood pressure. A clip-on probe sends an IR beam through the fleshy tissue and receives reflections therefrom. Changes in the amount of blood in the measurement area (i.e., a capillary bed) cause changes in absorption or variation, and such changes vary along with the amount of blood delivered to that tissue. Thus, although not a direct measure of the patient's blood pressure, the plethysmographic signal emulates the waveform contour and magnitude of direct intra-arterial pulse pressure and is typically displayed on a monitor screen along with the electrocardiogram and respiratory excursions. Clinical use of this measurement, called plethysmographic oximetry (PO), has been reported to detect pulsus paradoxus in children with pericardial tamponade and in adults with respiratory distress from obstructive lung disease but these results have not yet been incorporated into devices that would make possible simply applied, non-invasive, real-time detection of pulsus paradoxus.
The above described techniques for detection of pulsus paradoxus are highly subjective, labor intensive and inaccurate (if measured with a blood pressure cuff), or invasive (if done with arterial lines) and therefore, are rarely used. Thus, the ability to detect the ominous condition of pulsus paradoxus requires a high level of suspicion and cumbersome or invasive technology. As a result, pulsus paradoxus and therefore early signs of several life-threatening conditions often go undetected.